What is the meaning of “apostle”?

The Greek word that is translated apostle is apostolos, which simply means messenger, one sent forth.  It is not a special ecclesiastical term.  This is evident as sometimes apostolos is translated as sent, for example in John 13:16.

Joh 13:16  Most assuredly, I say to you, a servant is not greater than his master; nor is he who is sent [apostolos] greater than he who sent him.

Sometimes apostolos is translated as messenger, for example in 2 Corinthians 8:23 and Philippians 2:25.

2Co 8:23  If anyone inquires about Titus, he is my partner and fellow worker concerning you. Or if our brethren are inquired about, they are messengers [apostolos] of the churches, the glory of Christ.

Php 2:25  Yet I considered it necessary to send to you Epaphroditus, my brother, fellow worker, and fellow soldier, but your messenger [apostolos] and the one who ministered to my need;

Apostles of Jesus Christ

Apostles of Jesus Christ or God are people sent by God.  There are only a few men in the Bible who are called apostles, and they are called apostles exactly because they were sent by God. 

The foremost apostle is Jesus Christ.

Heb 3:1  Therefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our confession, Christ Jesus,

Jesus Christ was sent by the Father.

Joh 4:34  Jesus said to them, “My food is to do the will of Him who sent Me, and to finish His work.

Jesus Christ was therefore an apostle of God the Father.

The 12 disciples were also called apostles.

Luk 6:13  And when it was day, He called His disciples to Himself; and from them He chose twelve whom He also named apostles:
Luk 6:14  Simon, whom He also named Peter, and Andrew his brother; James and John; Philip and Bartholomew; 
Luk 6:15  Matthew and Thomas; James the son of Alphaeus, and Simon called the Zealot; 
Luk 6:16  Judas the son of James, and Judas Iscariot who also became a traitor.

Jesus sent the disciples into the world, which is why He named them apostles.

Joh 20:20  When He had said this, He showed them His hands and His side. Then the disciples were glad when they saw the Lord.
Joh 20:21  So Jesus said to them again, “Peace to you! As the Father has sent Me, I also send you.”

The 12 disciples were therefore apostles of Jesus Christ.

Later Matthias was numbered with the apostles as a replacement for Judas Iscariot.

Acts 1:26  And they cast their lots, and the lot fell on Matthias. And he was numbered with the eleven apostles.

Paul and Barnabas were apostles.

Rom 1:1  Paul, a bondservant of Jesus Christ, called to be an apostle, separated to the gospel of God

Acts 14:14  But when the apostles Barnabas and Paul heard this, they tore their clothes and ran in among the multitude, crying out

When God told Ananias to visit Paul after he had been struck down on the road to Damascus, God explained that He was sending Paul to preach to the Gentiles.

Act 9:15  But the Lord said to him, “Go, for he is a chosen vessel of Mine to bear My name before Gentiles, kings, and the children of Israel.
Act 9:16  For I will show him how many things he must suffer for My name’s sake.”

Paul was sent by the Lord.  Therefore, Paul was an apostle of Jesus Christ. 

James the brother of Jesus was also called an apostle.

Gal 1:19  But I saw none of the other apostles except James, the Lord’s brother.

Although the Scriptures do not explicitly state that Barnabas and James were sent by God, it’s a fair assumption as they both did the work of an apostle. 

The Bible does not explicitly say that Silvanus and Timothy were apostles, but it’s certainly a possibility.

1 Th 1:1  Paul, Silvanus, and Timothy, To the church of the Thessalonians in God the Father and the Lord Jesus Christ: Grace to you and peace from God our Father and the Lord Jesus Christ.

1 Th 2:6  Nor did we seek glory from men, either from you or from others, when we might have made demands as apostles of Christ.

Likewise, Andronicus and Junia could possibly have been apostles.  Does “are of note among the apostles” mean they were apostles or simply gained the respect of the apostles?

Rom 16:7  Greet Andronicus and Junia, my countrymen and my fellow prisoners, who are of note among the apostles, who also were in Christ before me.

The signs of an apostle

The Bible shows that a true apostle of Jesus Christ will have certain signs.

2 Cor 12:12  Truly the signs of an apostle were accomplished among you with all perseverance, in signs and wonders and mighty deeds.

The signs of an apostle are, signs, wonders and mighty deeds.

Act 2:43  Then fear came upon every soul, and many wonders and signs were done through the apostles.

Act 5:12  And through the hands of the apostles many signs and wonders were done among the people. And they were all with one accord in Solomon’s Porch.

The apostles did many signs and wonders.  They were miraculous healings and casting out of demons.

Act 5:15  so that they brought the sick out into the streets and laid them on beds and couches, that at least the shadow of Peter passing by might fall on some of them.
Act 5:16  Also a multitude gathered from the surrounding cities to Jerusalem, bringing sick people and those who were tormented by unclean spirits, and they were all healed.

Conclusion

An apostle of Jesus Christ is someone who has been sent by Jesus Christ and performs the signs and wonders of an apostle, such as miraculous healings and casting out of demons.  

Footnote

Are there modern-day apostles of Jesus Christ?  Was, for example, Herbert W. Armstrong an apostle?  If they were sent by Jesus Christ and have the signs of an apostle, then yes.  If they claim to have been sent by Jesus Christ but do not have the signs of an apostle, then it seems most unlikely.